Aspects of this disclosure relate generally to communication systems, and more particularly to asynchronous bias control for an optical modulator.
Optical signals are increasingly used to communicate data in telecommunication systems. Optical communication systems are capable of high-speed data transfer. Moreover, optical signals are generally of high quality because, for example, they are not distorted by electromagnetic fields.
Optical modulation systems may be used to convert an electrical data signal into a phase-modulated and amplitude-modulated optical signal. However, optical modulation systems generally rely on bias control to optimize performance. To control bias, the optical modulation system may include a drive circuit that generates a low-frequency dither signal. The optical modulation system superimposes the generated dither signal onto the electrical data signal. The electrical data signal (having the dither signal superimposed thereon) is then provided to the optical modulator and converted to an optical data signal.
An optical modulator preferably operates at a null transmission point. Any deviation from the null transmission point may lead to degradation of the optical data signal. This deviation may be referred to as a bias ϕ. For example, the transmission point of the optical modulator may shift due to environmental factors (for example, excess heat) or due to aging of the optical modulator.
To control the bias ϕ, the optical modulation system may generate a feedback signal. In particular, the optical modulation system taps a portion of the optical data signal generated by the optical modulator. The optical modulation system further includes a detector circuit that recovers the dither signal from the tapped optical data signal. The detector circuit then compares the recovered dither signal obtained from the optical data signal to the generated dither signal generated by the drive circuit. Based on the comparison, the detector circuit can determine the bias ϕ and generate a feedback signal for transmission to the optical modulator. The optical modulator may then adjust the transmission point based on the feedback signal.
As a practical matter, the value of the bias ϕ may be obscured by a delay τ caused by processing of the dither signal (for example, processing and conversion delay as the dither signal passes through the optical modulator). For ease of comparison, a synchronous optical modulation system may provide the generated dither signal to the detector circuit, thereby synchronizing the drive circuit with the detector circuit. Because the optical modulation system is synchronized, the detector circuit can account for any delay (τ) caused by processing or conversion of the dither signal as it passes through the optical modulator. Once the delay τ is accounted for, the bias ϕ can be determined independently of the delay τ.
However, in asynchronous optical modulation systems the dither signal generated by the drive circuit is not provided to the detector circuit. Accordingly, a need arises for generating a feedback signal that is independent of the delay τ without relying on synchronization.